Scaling problem in Parkinson's disease patients with pain

Introduction: Although pain is common in Parkinson's disease (PD), the underlying mechanism remains unknown. Scaling function and dopaminergic hypofunction may contribute to pain development because increased pain sensitivity is observed in PD and is normalized after levodopa administration. We aimed to determine whether spatial discrimination (SD) and striatal dopaminergic activity (DA) differed between PD patients with and without pain. Methods: We divided 90 patients with drug-naive PD into two groups based on the presence or absence of pain and compared the SD threshold (SDT). We evaluated the correlation of the SDT with pain severity in PD with pain. We also compared the DA of 48 patients and analyzed the correlation with pain severity in PD patients with pain. Results: The SDTs did not differ between the two groups, but unmeasurable SDT was more frequent in PD with pain. There was a positive correlation of pain severity with the SDT of the more affected hand but no correlation with the SDT of the less affected hand. The DA did not differ between the groups. There was a negative trend of pain severity with the DA of the ventral striatum (VS) but no correlation with the other striatal subregions. Conclusions: Pain in PD may be associated with scaling dysfunction in the sensory system. The abnormal scaling function would render the PD patient hypersensitive to even mild pain. The dopamine in the VS appears to be associated with pain severity; however, the relationship of striatal dopaminergic deficits with pain occurrence requires further investigation.